Method for measuring the health of solid rocket propellant using an embedded sensor

ABSTRACT

The invented method for measuring the health of a solid rocket propellant includes embedding at least one piezoelectric capacitance sensor in the propellant, where the capacitance of the sensor is a function of a modulus of the propellant, and where the position is selected to measure manifestations of stress failure as a consequence of changes in the shear modulus. The capacitance of the sensor is measured at a predetermined frequency. The capacitance of the piezoelectric capacitance sensor is converted into a digital representation which is then converted into the digital representation of a modulus (or gradient modulus). In subsequent analysis, the modulus (or gradient of the modulus) is correlated to the health of the solid rocket propellant.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for Governmental purposeswithout the payment of any royalties thereon or therefore.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to sensors, and in particular toa nondestructive method of utilizing piezoelectric sensors to determinein situ the health of solid rocket propellants.

There is a need for a method of measuring the health of solid rocketpropellant using an embedded sensor for self-sensing ordnance. The needis particularly acute with regard to solid rocket motors, since it isknown that aging of propellant can lead to significant degradation inweapon performance and, possibly, catastrophic failure. The needcorrelates with the military requirements that mandate future tacticalmissiles be kept for extended periods of time on board ship, withoutperiodic, land-based inspections. A particularly facile method ofinspection would be one where the solid rocket missiles have a solidrocket motor with a propellant that has a method of self-sensing thehealth of the propellant.

Classical approaches used to predict and detect material degradationhave been to develop aging models for predicting the state of amaterial, given an assumed or measured environmental history, and theuse of non-destructive testing methods such as ultrasound and X-rays.Both approaches, as currently practiced, are inadequate to meet theneeds of a real-time, self-sensing health monitoring system. Thus, inrecent years efforts have been devoted to investigate an entirely newapproach to meet the goal of self-sensing ordnance—the use of embeddedsensors.

The use of embedded sensors is potentially a better analytical techniquefor several reasons. A sensor embedded in the propellant inside aweapon, versus an external technique that is obstructed by a thickmetallic housing, is in direct contact with the energetic material, andthus in a better position to detect subtle changes in properties. Anembedded sensor is always present in the weapon, and thus the weapon'shealth can always be queried, thus meeting the goal of making theordnance self-sensing.

Several types of embedded sensors are being investigated in the solidrocket motor community. Bond line sensors are small pressure sensorsused to measure the stress between the propellant and case. The sensorsare used to detect the perturbation in the stress field due to thepresence of damage. Difficulties with this method are as follows. Thebond line sensor requires sophisticated finite element modeling andanalysis to characterize the damage from the measured signals. Theanalysis is further complicated by the unknown change in materialproperties due to aging, leading to problems in data interpretation.Bond line sensors are difficult to install, as they need to be cast intothe rocket motor, and they have relatively high cost, on the order of$250 per sensor. A second type of sensor is an optical fiber strainsensor. Optical fiber strain sensors are used in a similar manner tobond line sensors, in that they detect changes in the strain field dueto the presence of damage. While optical fiber sensors can be placed inthe bore, and thus can be installed after the motor is cast, thedifficulty in interpreting the signals is a significant challenge.

A weakness with both these sensors is that they do not provide anunambiguous indication of the system's health, and significant analysisis needed to interpret the results.

What is needed is a reliable measure of the health of the solid rocketpropellant through the use of an embedded sensor, where the methodprovides an unambiguous measure of material state, where the methodemploys an embedded sensor that is relatively inexpensive.

SUMMARY OF THE INVENTION

The invention is a method of determining the health of solid rocketpropellants using an embedded piezoelectric capacitance sensor. Incontrast to prior methods using in situ sensors, which provide a measureof only the stress or only strain, the invention measures the modulus atpotentially multiple locations. By measuring the modulus at multiplelocations using multiple sensors one can determine gradients in modulus.The measurements can be made using a single integrated circuitinterrogator, which has the capability of determining the capacitance ofmultiple sensors. The capacitance of the sensors is then correlated to amodulus. Both ceramic and polymeric film piezoelectric sensors have areduction in capacitance as material in intimate contact with the sensorstiffens (e.g. the modulus increases). The ceramic piezoelectric sensorsare generally comprised of lead zirconate titanate (PZT). Piezoceramicsare very efficient, and are thermally stable, but have a poor mechanicalimpedance match to propellant and are brittle. The polymeric sensor maybe comprised of PVDF (polyvinylidene fluoride) or copolymers thereof.The PVDF piezoelectric sensors generally have a film of PVDF that isformed by stretching it below the melting point of the PVDF. Thestretching may be performed in the presence of a very high electricfield thereby imparting crystallinity, and highly orienting the C-Fpolymeric material under the influence of the electric field. The PVDFfilm often has an electrically conductive coating on one or both sides,where the conductive coating is for example selected from silver,nickel, aluminum, copper, gold, or other conductive alloys. The PVDFcrystalline film is a piezoelectric material and a dielectric materialthat is excellent for forming capacitors. While the piezoelectricpolymer has a better impedance match with propellant, and has theadvantage of being flexible, it has the weakness of relatively poorthermal stability, and therefore is unsuitable if during the embeddingprocess, the molten cast propellant is hot, as the heat can have adeleterious effect on the polymer (i.e. the polymer relaxes). It shouldbe noted that measuring the electrical impedance of the sensor is alsoof use, and the word “capacitance” is used to refer to electricalimpedance.

The method for nondestructively remotely measuring the health of anenergetic material includes embedding at least one piezoelectriccapacitance sensor in the energetic material, wherein the capacitance ofthe sensor is a function of a modulus of the energetic material;periodically measuring the capacitance of the piezoelectric capacitancesensor; converting the measurement of the capacitance into a digitalrepresentation; communicating the digital representation to a remotecommunication device; relating the digital representation to modulus;and correlating the modulus to the health of the energetic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the capacitance versus cure time for a resin as itstiffens, increasing in modulus, using a PVDF sensor;

FIG. 2 is a diagrammatic view of solid rocket propellant motor having aplurality of embedded piezoelectric sensors in communication with aninterrogator, where power and query command is housed in the guidancesection of the rocket; and

FIG. 3 a diagrammatic view of a pill-sized unit that has self-containedinterrogation and communication electronics, where the unit has at leastone integral piezoelectric sensor, and can be embedded during thecasting of the propellant.

DETAILED DESCRIPTION

The invention is a method, and related apparatus, for nondestructivelyremotely measuring the health of an energetic material, where theenergetic material is a solid rocket propellant. The invented methodrecognizes that as propellants and the like develop health issues, thematerials become harder (i.e. shear modulus increases), and there can bepropellant/insulater debonding and bore cracking. Cracks and debondingcan induce failure of the rocket upon ignition. In addition, somepropellants may become softer with age, leading to “slump”—unacceptablepermanent deformation of the propellant. Motors are chemically activethroughout their entire lives, leading to issues of motor aging perhapsfar down the road. A major cause of hardening in propellants over timeis oxidative cross-linking. The introduction of moisture into the systemmay also be extremely destructive to material properties in both thebulk materials and at the interfaces. There are two general classes offlaws in the PLI system. The first is a void or inclusion, generallylocated in the bulk propellant. The second is a fracture or debond.Voids in propellant often occur as a result of insufficient settling ofthe propellant during the casting process. Trapped air bubbles are notfully eliminated and small voids are formed which contain no propellant.If small enough, these small voids are not generally of great concern.However, if the voids are proximate to an interface or other high stressor strain region, then these small voids may contribute to the formationof cracks. Inclusions are objects that inappropriately end up in thepropellant. Inclusions may be large pieces of propellant ingredients orother motor materials, but also include anomalous objects. Notableinclusions that have appeared in motors include lead shot, a crumpledpaper cup, and a wrench. Regardless of the source, these objects areoften poorly bonded to the propellant and cause perturbations to thestress/strain field of the motor in a similar fashion as voids. If theitem is large enough or is likely not to be fully consumed in the motor,then the item can damage the housing and the nozzle. Depending on thecomposition of the inclusion, the combustion process in the region canbe significantly changed. In some cases, materials such as fine metalwires are placed in the propellant to increase the burning rate byaugmenting thermal conduction and providing a flame path.

Cracks can occur throughout the motor, although they are often seen inthe bore, particularly in motors that have undergone thermal cycling.When a crack occurs, there are two scenarios. In the first case, whenthe combustion surface reaches the crack, the flame speed exceeds thecrack propagation velocity. In this situation, the crack tip is bluntedby the burning and does not propagate, so the concern is simply theincrease in pressure of the motor due to additional burning surfacearea. If the crack area is small compared to the surface area of themotor, the pressure will not be significantly increased and this willnot a major issue. In the case that the crack propagation speed isgreater than that of the flame, the crack will propagate. In thissituation, burning surface is exposed deeper in the motor before it wasexpected. Since the insulation thickness is determined by the time ofexposure to the hot gases (with an appropriate factor of safety), earlyexposure can overwhelm the insulation, heating the housing and creatingan opportunity for failure. Cracks also occur in the propellant near thepropellant-liner interface. These cracks compound the problem, as notonly is there hot gas near the wall, but if the crack propagates, itdetaches the motor grain from the bonding surface. Debonds are similarto the cracks described above, but result from insufficient orincomplete bonding between two of the propellant-liner-insulatormaterials. As with cracks, the concerns are augmented burning near thehousing wall and the structural impact of the decreased bonding.

In any case, the degree of degradation in the propellant can be detectedby a change in capacitance, as the previously enumerated symptoms aregenerally manifestations of stress failure as a consequence of changesin the shear modulus. FIG. 1 illustrates the capability of PVDFpiezoelectric sensor to detect an increase in stiffness. As thesurrounding material becomes stiffens with time, there is a decrease incapacitance. In the illustration, the increase in stiffness (i.e.modulus) is due to cross-linking, and is a illustrative of themeasurement capabilities of piezoelectric sensors. The historical dataand auxiliary test data, such as X-ray data should be factored into theinstallation of the embedded sensors.

Deciding on the position of the sensors depends on whether they will beused mainly for determination of modulus or damage. The two typicallycorrelate, but can vary in degree. If the primary use is to determinemodulus, then the sensors are placed in a low stress area, so that itwill be unlikely that the volume of material surrounding the sensor willcontain damage. Example areas would be in locations far from the bulbtip stress reliefs. If the primary use will be to determine whetherdamage is present (damaged material also can be considered as “lessstiff material”), than the sensors should be placed near areas wheredamage is expected to occur, such as near bulb tip stress reliefs andthe propellant-liner interface.

Monolithic sensors can be used to generate both normal and tangentialmotion. A sensor with a composite construction also allows both types.As propellants are very energetic materials, the piezoelectric sensor isselected to develop very little heat to minimize the possibility ofaccidental ignition. A rough order of magnitude calculation shows thatthe temperature rise due to powering the sensor is minimal. Assume thatthe area of sensor is 1 mm², the capacitance of sensor is 0.32 nf, theexcitation voltage is 10 mv, the frequency of excitation is 32 KHz, thevolume of material surrounding sensor is 10⁻³ cm having a mass of 1 mg,and heat capacity of propellant of 0.5 cal/g ° C.; then the powerdissipated by the sensor is about 0.2 microwatt, and the temperaturerise in the propellant surrounding the sensor would thus be about3×10⁻⁵° C./sec. This temperature is within a large margin of safety.

The invented method further discloses how energy is provided for thesensor and the support electrical components. The method also discloseshow capacitance is measured, and how the measured capacitance iscommunicated from the embedded sensor to a monitoring system, such asATOS, which is an advanced technology ordnance surveillance system.

The invented method for measuring the health of a solid rocketpropellant includes the steps of embedding at least one piezoelectriccapacitance sensor in the propellant, where the capacitance of thesensor is a function of a modulus of the propellant, and where theposition is determined by the previously enumerated considerations. Thesensor's capacitance is measured at a predetermined frequency. Thecapacitance of the piezoelectric capacitance sensor is converted into adigital representation which is communicated to a remote device thatconverts the digital representation to a modulus or gradient in modulusgradient (when multiple sensors are used). In subsequent analysis, themodulus (or gradient in modulus) is correlated to the health of thesolid rocket propellant. The piezoelectric capacitance sensor includesone sensor or a plurality of sensors embedded in the energetic material,where individual sensors can be queried. As illustrated in FIG. 2, thesensors 12 are electrically attached to an integrated circuit 10, wherethe integrated circuit is in communication with an microcontroller 14,which records and processes the information. The sensors 12 asillustrated are embedded in the propellant 62 of rocket 60 having amotor 68 and a guidance section 66. The motor 68 is separated from theguidance section 66 by bulkhead 64. A thick metallic housing 68 encasesthe propellant 62, which has a substantially annular burning bore 70.

Examples of a single chip capacitance analog to digital converters areAD7745 (single input) or AD7746 (dual input) chips, which are productsof Analog Devices, Inc. AD7745/AD7746 are high resolution, Σ-Δcapacitance-to-digital converters (CDC). The capacitance to be measuredis connected directly to a sensor. The CDC architecture featuresinherent high resolution (24-bit no missing codes, up to 21-biteffective resolution), high linearity (±0.01%), and high accuracy. Thecapacitance input range is ±4 pF changing), while it can accept up to 17pF common-mode capacitance not changing), which can be balanced by aprogrammable on-chip, digital-to-capacitance converter (CAPDAC). TheAD7745 chip has one capacitance input channel, while the AD7746 chip hastwo channels. Each channel can be configured as single-ended ordifferential. The CDCs interrogators are designed for floatingcapacitive sensors. The chips have an on-chip temperature sensor with aresolution of 0.1° C. and accuracy of ±2° C. The chips also have anon-chip voltage reference and an on-chip clock generator, and theseeliminate the need for any external components in capacitive sensorapplications. The chips have a standard voltage input, which togetherwith the differential reference input allows easy interface to anexternal temperature sensor, such as an RTD, a thermistor, or a diode.The CDCs can operate with a single power supply from 2.7 V to 5.25 V.Alternatively, a series of sensors can be interrogated using a CDC suchas AD7142, which can sample up to 14 sensors. The AD7142, which is alsoa product of Analog Devices, Inc, is an integratedcapacitance-to-digital converter with on-chip environmental calibrationfor use in systems requiring a novel user input method. Although thesensor excitation frequency of the AD7142 is fixed, the resonancefrequency of the sensors may be tailored to match this frequency, ifnecessary. The AD7142 CDC has 14 inputs channeled through a switchmatrix to a 16-bit, 250 kHz sigma-delta (Σ-Δ) capacitance-to-digitalconverter. The CDC is capable of sensing changes in the capacitance ofthe external sensors and uses this information to register a sensoractivation. The AD7142 has on-chip calibration logic to account forchanges in the ambient environment. Another integrated circuitinterrogator is the AD5933, which measures the electrical impedance ofthe sensor, thus providing phase information in addition to capacitance.An alternative method for measuring capacitance includes placing eachsensor in a voltage divider configuration with a fixed resistor.Measuring the voltage across the resistor to provide a measure of boththe capacitance (through the impedance-capacitance relationship for acapacitor) and the current through the sensor. Accordingly, thecurrent-voltage relationship may be obtained and of use.

As illustrated in FIG. 2 the interrogator 14 is in communication with adigital to optical converter 16, which generally includes amicrocontroller. The digital to optical converter 16 converts thedigital representation into an optical signal which is transmittedthrough a fiber optic cable 20 to an optical to digital converter 22located in the guidance section 66 of the rocket 60. Information isuploaded to an RFID device 24, which is a scannable member of ATOS(advanced technology ordnance surveillance system). As illustrated inFIG. 2, the optical fiber 20 can additionally provide a non-electricalmeans of transmitting power through the innards of the rocket motor 68.The optical to digital converter 22 can send enough light power throughthe optical fiber 20 to power the digital to optical converter 16 andthe CDC interrogator 14. The optical fiber 20 interfaces an optical tovoltage converter 18.

The method can employ a piezoelectric capacitance sensor selected fromthe group consisting of a piezoceramic sensor or a piezoelectric polymersensor. Generally, the piezoelectric polymer sensor is comprised ofpiezoelectric PVDF polymer, copolymer, or a combination thereof.Generally, the piezoceramic sensor is comprised of lead zirconatetitanate.

FIG. 3 illustrates a diagrammatic view of a pill-sized unit that hasself-contained interrogation and communication electronic components,where the unit has at least one integral piezoelectric sensor, and canbe embedded during the casting of the propellant. In the inventedmethod, the embedded sensor apparatus 10 includes a series of sensors12, which interface an RF communication component 25 housed in aprotective can 50. The size of the pill-sized unit may be on the orderof about 2 mm-20 mm, and more particularly, about 5 mm. The pill-sizedunit contains the CDC 14, such as the AD7142 previously discussed; a lowpower microcontroller 16, such as 8051F300 (e.g. 8051); a battery 52;and the RF communication chip 25, such as SX1223. SX1223 is a product ofSemtech. The SX1223 is a single chip transmitter operating in the UHFfrequency bands including the 434, 869 and 915 MHz license-free ISM(Industry Scientific and Medical) bands. Its highly integratedarchitecture allows for minimum external components while maintainingdesign flexibility. All major RF communication parameters areprogrammable and most of them can be set dynamically. The SX1223 offersthe advantage of high data rate communication at rates of up to 153.6kbit/s. The SX1223 is optimized for low cost applications while offeringhigh RF output power.

The method for nondestructively remotely measuring the health ofpropellant in a solid rocket motor includes the steps of providing anelectronically integrated combination of piezoelectric capacitivesensors and communication components in a self contained, pill-sizedunit, where the sensors are in electrical communication through aprotective housing with the communication components. The communicationcomponents include a capacitance to digital converter; a low powermicrocontroller; an RF communication chip; and a battery. The methodfurther includes positioning the self contained, pill-sized unit in amold for the solid rocket propellant; and casting the solid rocketpropellant. The process further consists of periodically measuring thecapacitance of each of the piezoelectric capacitance sensors; convertingthe measurement of the capacitance into a digital representation;communicating the digital representation to a remote device; relatingthe digital representation to modulus (or gradient modulus); andcorrelating the modulus (or gradient modulus) to the health of thepropellant. The communication of queries, in an exemplary embodiment, iseffected via an RF communication chip, which is a member of an advancedtechnology ordnance surveillance system (ATOS). A stated purpose of theATOS is to locate and monitor the health of munitions.

To facilitate exact positioning of the sensors, the rocket motor casingcan be pre-fitted with scaffolding that holds the sensors prior tocasting the propellant. This method includes providing a rocket motorhousing with a scaffolding; attaching at least one piezoelectriccapacitance sensor to the scaffolding; casting the propellant, thereinforming a rocket motor with an annular bore 70, where the rocket motorhas at least one piezoelectric capacitance sensor embedded in solidifiedpropellant; connecting at least one piezoelectric capacitance sensor toa capacitance to digital converter, such as an interrogator positionednear the annular bore of the motor; interfacing a microcontroller havinga digital to optical converter with the capacitance to digital converterand with an optical fiber, where the optical fiber provides bothcommunication and power; extending a length of the optical fiber throughthe annular bore into the guidance section of the rocket; andinterfacing the optical fiber to a communication device. In an exemplaryembodiment, the communication device is an active RFID, which is ascannable member of the advanced technology ordnance surveillancesystem. In an exemplary embodiment, the scaffolding is a compliantmaterial (e.g. rubber), and has the capability of attaching andpositioning multiple sensors. The capacitance of individual sensors canbe queried using an interrogator.

Another method of installing the health monitoring system is by simplydropping the “pill” into the motor as it is being cast. The position canlater be determined using X-rays.

It is to be understood that the foregoing description and specificembodiments are merely illustrative of the best mode of the inventionand the principles thereof, and that various modifications and additionsmay be made to the invention by those skilled in the art, withoutdeparting from the spirit and scope of this invention, which istherefore understood to be limited only by the scope of the appendedclaims.

Finally, any numerical parameters set forth in the specification andattached claims are approximations (for example, by using the term“about”) that may vary depending upon the desired properties sought tobe obtained by the present invention. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, each numerical parameter should at least beconstrued in light of the number of significant digits and by applyingordinary rounding.

1. A method for nondestructively remotely measuring the health of anenergetic material, comprising: embedding at least one piezoelectriccapacitance sensor in the energetic material, wherein a capacitance ofsaid at least one piezoelectric capacitance sensor is a function of amodulus of the energetic material; periodically measuring thecapacitance of said at least one piezoelectric capacitance sensor;converting the measurement of the capacitance into a digitalrepresentation; communicating the digital representation through anactive RFID tag to a remote communication device; relating the digitalrepresentation to the modulus; and correlating the modulus to health ofthe energetic material, wherein said active RFID tag includes aradio-frequency optical output and optical input, said optical input inoptical and power communication through an optical fiber to an opticalvoltage converter in communication with a micro-controller and digitalto optical converter, which is in communication with a interrogator. 2.The method according to claim 1, wherein said at least one piezoelectriccapacitance sensor is a plurality of sensors embedded in the energeticmaterial, where individual sensors of said plurality of sensors arequeried.
 3. The method according to claim 2, wherein the plurality ofsensors are elements of a circuit each having a respective saidcapacitance, and wherein said circuit is in communication with saidinterrogator, which is a capacitance to digital converter interrogator.4. The method according to claim 2, wherein the plurality of sensors areelements of a circuit each having a respective said capacitance, andwherein the capacitance of each of the individual sensors in the circuitis measured at a predetermined frequency using said interrogator.
 5. Themethod according to claim 1, wherein the RFID tag is a scannable memberof an advanced technology ordnance surveillance system (ATOS), where anapplication of ATOS is to locate and monitor the health of munitions. 6.The method according to claim 1, wherein said at least one piezoelectriccapacitance sensor is selected from one of a piezoceramic sensor and apiezoelectric polymer sensor.
 7. The method according to claim 6,wherein said piezoelectric polymer sensor is selected from at least oneof a piezoelectric PVDF polymer and a copolymer.
 8. The method accordingto claim 6, wherein said piezoceramic sensor is lead zirconate titanate.